The refractory metal borides, carbides and nitrides have already been proposed for use as components of electrolytic aluminum production cells, particularly those components which in use may normally be contacted with molten aluminum, such as an electrowinning cathode or cathode current feeder. Borides and in particular titanium diboride are often preferred on account of their excellent wettability by molten aluminum, good electronic conductivity and so forth. However, many problems have been encountered in producing these materials with a sufficient resistance to attack by molten aluminum and by the conventional alumina-containing cryolite-based melts used in aluminum electrowinning cells.
It is known to produce borides by the carbothermic reduction of oxides e.g. titanium oxide and boron oxide, or by chemical reduction with alkali metals, magnesium or aluminum. Typically, titanium dioxide and boric anhydride, or titania hydrate and boric acid are used as starting materials and the product is a boride powder. These boride powders are relatively impure and when they are formed into a body e.g. by hot pressing or sintering, the bodies are prone to severe grain boundary corrosion when exposed to molten aluminum. For example, UK Patent Specification No. 1004585 discloses the production of TiB.sub.2 by reacting carbon with anatase or rutile titanium dioxide in a glassy matrix of boron oxide formed from boric acid. U.S. Pat. No. 3,379,647 describes boride production from intimately mixed reactants including boric acid or borax as a boron source, a carbon source, and an unspecified source of an oxide of a boride-forming metal. However, in both instances, full reaction of the components is unlikely and contamination of the products with unreacted reactants makes them unsuitable for many uses such as components of electrolytic aluminum production cells which contact the molten aluminum.
Another way of making titanium diboride articles is by heating pellets of boron carbide (B.sub.4 C), carbon and rutile titanium dioxide, grinding the resulting titanium diboride pellets into a powder and shaping and sintering the powder. However, the grinding operation is expensive and necessarily introduces impurities due to the abrasive nature of B.sub.4 C thereby making the resulting articles non-resistant to contact with molten aluminum.
The production of submicronic boride powders by vapor phase reaction of chlorides has been proposed in U.S. Pat. Nos. 4,282,195, and these submicron powders can be sintered into bodies. This process is relatively complicated and the powder handling, pressing and sintering operations necessarily introduce impurities.
It is also known to produce borides by electrolysis of a melt typically containing the metal oxide and boron oxide or borax with a flux of alkali or alkaline-earth halides or fluoroborates. This process is however limited mainly to coatings and is relatively expensive.